How can I position bones in a model easily and precisely?
Quick and Precise Bone Placement using Snap To

Positioning bones in a model can be quickly and easily achieved using two of Blender's Snap To commands: 


Snap Selection to Cursor: this moves/snaps the selected object to the 3D Cursor (  )
SHIFTS -> Snap Selected to Cursor
Snap Cursor to Selected: this snaps the 3D Cursor to the selected object
SHIFTS -> Snap Cursor to Selected




For this method of bone placement, you will also want to enable two Interface preferences:  


Zoom to Mouse Position: When enabled, the mouse pointer position becomes the focus point of zooming, instead of the 2D window center. 
Rotate Around Selection: When enabled, the selected object becomes the rotation center of the viewport.

 


One other option that is very important is to make sure you can see your rig's bones even when they are inside the mesh. This is done by enabling X-Ray for your rig:  



With these preferences enabled, you can now very easily examine your figure's mesh and begin to position the bones. 

Positioning the Bones

There are 3 parts of a bone: 


the Head (or Root)  
the Body
the Tail (or Tip)




The key to this technique is to: 


Find and select 2 vertices on opposite sides of your mesh's surface where you want to position the bone's tail:  

  

NOTE: In order to see through the mesh, make sure "Limit Selection to Visible" is disabled:

Now, with those 2 points selected, use Snap 3D Cursor to Selected
SHIFTS -> Snap Cursor to Selected 

 

This positions the 3D Cursor directly between the two selected vertices, exactly where we want to put the bone tail.
Now select the bone tail and snap it to the 3D Cursor:  

 
Now we want to position the head of the bone at the tip of the finger. To do this, we select one vertex on the fingertip and snap the cursor to it:  


Next, we snap the bone head to the 3D cursor:  



The bone is now perfectly positioned.